Rising CO2 concentration ([CO2]) in the atmosphere may modulate the response of crop plants to drought stress. This study aimed to investigate the response of leaf gas exchange and plant growth of two C4 species representing both dicot (amaranth) and monocot (maize) to progressive drought under two different [CO2] (ambient (a[CO2], 400 ppm) and elevated (e[CO2], 800 ppm)). The soil water status in the pots was expressed as the fraction of transpirable soil water (FTSW). The results showed that as compared to a[CO2], e[CO2] significantly increas...